I've not use a cock-ring per se, in that I've not used a solid ring. However, in one or my previous relationships my partner did once use a piece of soft material to tie up my cock and balls.
How did it work? From memory she tied around the base of my cock and balls, then the base of my cock before separating each of the balls. By the time she had tied me, I certainly had a very strong erection.
Did it keep you hard for an extended time? I seem to remember remaining hard for much longer than normal, but we were very careful not to leave the tie in place for too long, as she'd read that that can be dangerous and lead to penile damage.
Did it prevent you from ejaculating? It didn't prevent me from ejaculating, although I seem to remember that the build-up of pressure before I finally ejaculated seemed more than usual; resulting in a more powerful, but shorter ejaculation.
Were there any side effects? don't remember any side-effects; although did hae some discomfort in balls for few hours afterwards, and tying and un-tying was slightly uncomfortable due to hairs getting trapped
Did you enjoy it? It was certainly a different experience.
Did your partner enjoy it? I believe so.